Currently in situations where the number of end users exceeds network capacity, which may occur in emergency situations or for large public gatherings, such as sporting events or New Years Eve celebrations, the telecommunications network can quickly become congested. To allow high priority users, such as the emergency services, to get through, “Access Class Barring” is typically utilised. Access Class Barring is used to restrict public usage of the telecommunication resources.
To implement Access Class Barring, all mobile devices have an access class, which is a numerical identifier associated with the mobile terminal's SIM card. The SIMs used by high priority users each have an access class of between 11 and 15, and the SIMs used by the public are allocated an access class of between 0 and 9. The status of an additional Access Class 10, common to all access classes, determines whether or not a particular mobile terminal (with or without a SIM card being present) may make an emergency call. The status is signalled over the air to the mobile terminal by the home or visited Public Land Mobile Network (PLMN). In the time of an emergency, access to the base stations in the problematic geographic region will be restricted by barring a number of classes of mobile terminals from using the network within that geographic region. This restriction could range from a full exclusion of the public, to only one or two public priority classes.
As the emergency services do not usually require all the network resources to be dedicated to them, it is typical that only a subset of the public classes are excluded from network access in the problematic cells at any one time. This is achieved by periodically cycling through all the public classes, for instance, classes 0 to 2 may be allowed network access for a period of time, and then classes 3 to 5 for a subsequent period of time and so on. It is important for the general public to have some ability to communicate in emergency situations, particularly since the inability to communicate can lead to greater levels of stress and panic, both on an individual basis and at a crowd level.
In practice, each cell broadcasts a list of access class groups which are allowed to access the cell. At the initiation end, the terminal of the initiating party will check on the SIM for the access class groups which the subscriber belongs, If the subscriber does not belong to one of the access class groups which is currently allowed on the cell at that time the terminal informs the subscriber that the call set up request is denied and no communication is established between the terminal and the BTS.
At the terminating end, when an incoming communication is received for a subscriber in a congested and restricted cell, the terminating subscriber is paged and the terminal of that terminating subscriber will check on the SIM for the access class which the subscriber belongs. If the subscriber does not belong to one of the access class groups which is currently allowed on the cell at that time the terminal will not respond to the paging message. The network may attempt to page a couple of times, before informing the party initiating the call that the called subscriber is not reachable.
Access Class Barring is not an ideal solution for providing public access to the communication channels in the time of an emergency, particularly if the called party is in a restricted cell, as it is only the classes with access at any given time that are able to make and receive calls. The decision of whether to allow a communication is made based on whether each subscriber is permitted access to its local cell, regardless of the other party involved in the communication. If either subscriber is restricted by their local cells at the time of the communication set up request, the communication set up is prevented. In other words, for a subscriber-to-subscriber communication to be established, and both parties are within a restricted cell, then both parties need to be of the classes that are allowed to access the network. For example, considering the situation of classes 1, 5 and 9 only being given network access in a restricted cell, if a person with a mobile terminal in class 1 wishes to call a person with a mobile terminal in class 8, and both terminals are within the restricted cell, then while the class 1 terminal will be able to initiate the call, the class 8 terminal will not respond to the call, as that terminal does not currently have network access. Overall, if half of all public terminals have been barred access to the network in a restricted access area, then the probability of two users being able to successfully establish a communication, when both terminals are within the restricted area, is only one in four (i.e. only a 25% success rate).
It is also to be appreciated that another problem of Access Class Barring is that when a communication cannot be established, it results in wasted signalling resources, as a terminal excluded from network access in a restricted cell will not respond to the incoming call, despite the effort that has gone into tracking the terminal down. This needless paging can also create further unnecessary congestion in the restricted cell. Additionally this process wastes traffic resources on the cell at the initiating end, as the network allocates a traffic channel to this subscriber whilst the network tries to contact the terminating party.